Methods of forming crosslinked polyolefin nanocomposites having high wear resistance

ABSTRACT

Methods for forming polyolefin nanocomposite precursor compositions are provided. In embodiments, such a method comprises mixing a polyolefin, unmodified graphite, and a peroxide crosslinker via solid-state shear pulverization under conditions to form a polyolefin nanocomposite precursor composition comprising the polyolefin; exfoliated, unmodified graphite dispersed throughout the polyolefin; and unreacted peroxide crosslinker dispersed throughout the polyolefin, wherein the polyolefin is polyethylene, a copolymer of polyethylene, or combinations thereof. Methods of forming crosslinked polyolefin nanocomposites, the polyolefin nanocomposite precursor compositions, and crosslinked polyolefin nanocomposites are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 17/378,857 filed Jul. 19, 2021, which claimspriority to U.S. provisional patent application No. 63/053,997 that wasfiled Jul. 20, 2020, the entire contents of both of which areincorporated herein by reference.

BACKGROUND

Because of its many positive attributes, including flexibility, chemicalresistance and low cost, low-density polyethylene (LDPE) is widely usedfor packaging and a range of industrial and medical applications, amongothers. However, due to its poor wear and abrasion resistance, theapplication of LDPE under continuous friction is limited. One possiblesolution is to incorporate rigid filler into neat LDPE matrix in orderto improve the wear resistance.

However, the efforts to enhance the wear resistance of LDPE are limitedbecause the incorporation of nanofiller can only result in LDPEcomposites with moderate wear resistance. Consequently, most studiesaimed at achieving good wear resistance in polyethylene (PE)-basedmaterials have focused on another form of PE, ultra-high molecularweight polyethylene (UHMWPE), which exhibits superior wear resistanceeven in the neat state. Because of its low friction coefficient,chemical stability and biocompatibility, UHMWPE has been used fortribological contact pairs applications, such as artificial joints.However, due to the presence of extremely long chains and ultra-highmelt viscosity, the melt processability of UHMWPE is severely limited.

SUMMARY

Provided are methods for forming polymer nanocomposite precursorcompositions and crosslinked polymer nanocomposites formed therefrom.

Methods for forming polyolefin nanocomposite precursor compositions areprovided. In embodiments, such a method comprises mixing a polyolefin,unmodified graphite, and a peroxide crosslinker via solid-state shearpulverization under conditions to form a polyolefin nanocompositeprecursor composition comprising the polyolefin; exfoliated, unmodifiedgraphite dispersed throughout the polyolefin; and unreacted peroxidecrosslinker dispersed throughout the polyolefin, wherein the polyolefinis polyethylene, a copolymer of polyethylene, or combinations thereof.

Methods of forming crosslinked polyolefin nanocomposites are alsoprovided. In embodiments, such a method comprises subjecting apolyolefin nanocomposite precursor composition comprising a polyolefin;exfoliated, unmodified graphite dispersed throughout the polyolefin; andunreacted peroxide crosslinker dispersed throughout the polyolefin,wherein the polyolefin is polyethylene, a copolymer of polyethylene, orcombinations thereof, to a melt processing technique under conditions toinduce chemical reactions to crosslink chains of polyolefin, therebyforming a crosslinked polyolefin nanocomposite.

The polyolefin nanocomposite precursor compositions and crosslinkedpolyolefin nanocomposites are also provided.

Other principal features and advantages of the disclosure will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the disclosure will hereafter be describedwith reference to the accompanying drawings.

FIG. 1 shows a schematic representation of two-step process combiningSSSP and compression molding to prepare well-mixed crosslinkedLDPE/graphite nanocomposites.

FIG. 2 shows X-ray diffraction data of as-received graphite, (top) neatLDPE, (middle) LDPE/3G hybrid prepared by melt mixing in a cup-and-rotormixer and (bottom) LDPE/3G nanocomposites prepared SSSP. The intensitiesof neat LDPE and hybrids were normalized by making the area associatedwith PE crystal peaks equal. The highlighted area and inset are the areacontaining the characteristic diffraction peaks corresponding tounexfoliated graphite.

FIG. 3 shows non-isothermal crystallization curves (obtained uponcooling) for neat LDPE and 1DCP-LDPE and 3DCP-LDPE prepared by SSSP.(Cooling ramp is 10° C./min.)

FIGS. 4A-4C show wear track profiles of LDPE samples. FIG. 4A is a 2Dtop view of wear track on neat LDPE. FIG. 4B is a 3D view of wear trackon neat LDPE obtained via a 3D laser confocal microscope. FIG. 4C is thedepth profile of the wear tracks cross section (averaged over 300 μm inlength along the wear direction). The curves have been arbitrarilyshifted vertically for clarity. The hatched region in the profile istaken as cross section area for wear volume calculation.

FIG. 5 shows wear volume of the wear tracks in LDPE, 1DCP-LDPE (lightlycrosslinked) and 3DCP-LDPE (highly crosslinked) as a function of fillerloading. The error bar is associated with the standard deviation ofthree measurements.

FIGS. 6A-6B show the coefficient of friction in samples with variouscrosslinking density and filler loading. FIG. 6A shows the variation ofcoefficient of friction as a function of time in neat LDPE, 1DCP-LDPE(lightly crosslinked) and 3DCP-LDPE (highly crosslinked). FIG. 6B showsthe average coefficient of friction value over the entre duration of thetest in LDPE, 1DCP-LDPE and 3DCP-LDPE with various filler loading. Theerror bar is associated with the standard deviation of threemeasurements.

FIG. 7 shows conversion of crosslinking reaction under isothermalcondition monitored by differential scanning calorimetry.

FIG. 8 shows the depth profile of the wear track cross section in a neatcommercial UHMWPE (averaged over 300 μm in length along the weardirection). The hatched region in the profile is taken as cross sectionarea for wear volume calculation.

DETAILED DESCRIPTION

Provided are methods for forming polymer nanocomposite precursorcompositions and crosslinked polymer nanocomposites formed therefrom.The methods make use of solid-state shear pulverization (SSSP) toproduce polymer nanocomposite precursor compositions exhibitingversatile melt processability. The precursor compositions may then beused to provide crosslinked polymer nanocomposites, includingcrosslinked low-density polyethylene (LDPE) nanocomposites, exhibitingsurprisingly high wear resistance.

In embodiments, a method for forming a polymer nanocomposite precursorcomposition comprises mixing a polymer (e.g., a polyolefin), a nanoscalefiller, (e.g., unmodified graphite), and a crosslinker (e.g., aperoxide) via SSSP. SSSP is a continuous processing technique whichinvolves applying mechanical energy to a material (e.g., the polymer) inthe solid state (i.e., at a temperature below the melting temperature ofthe polymer of the precursor composition). Existing solid-state shearpulverizers (e.g., see FIG. 1 ) may be used to carry out SSSP andparameters such as screw design, screw speed, barrel size, and feed ratemay be adjusted to tune the conditions under which the mechanical energyis applied during SSSP. The SSSP is distinguished from melt mixingmethods which involve processing the material/polymer in its moltenstate. The temperature used during SSSP is below the melting temperatureof the polymer of the precursor composition. Thus, the temperature useddepends upon the particular polymer. However, in embodiments, thetemperature used is no more than 40° C., no more than 30° C., or aboutroom temperature (20° C. to 25° C.). Similarly, the temperature isgenerally below the dissociation temperature of the crosslinker, e.g.,the temperature at which the crosslinker dissociates into freeradical-containing fragments. SSSP does not require any solvents; thus,the precursor composition is generally free of a solvent.

In the present methods, the SSSP conditions are selected so that tworesults are achieved. First, the unmodified graphite nanoscale filler isexfoliated and dispersed into the polyolefin matrix. This means that thelayers of the unmodified graphite are separated and individual graphenenanoplatelets are homogeneously distributed throughout the polyolefinmatrix. Confirmation of exfoliation and dispersal of unmodified graphitemay be accomplished as described in the Example below, e.g., by usingX-Ray diffraction (XRD) to confirm the absence of a diffraction peak at26.5° (i.e., the normalized intensity at 26.5° is generally no greaterthan that shown in FIG. 2 , bottom curve). More specifically,exfoliation and dispersal of unmodified graphite may be confirmed byhaving a value of the normalized intensity at 26.5° for the precursorcomposition that is within 5% of the value of the normalized intensityat 26.5° for the neat polyolefin only (i.e., no unmodified graphite andno crosslinker). SSSP conditions to achieve exfoliated and dispersed,unmodified graphite are described in the Example below.

Second, the peroxide crosslinker is dispersed (i.e., homogeneouslydistributed) into the polyolefin matrix without dissociating theperoxide into free radical-containing fragments or otherwise inducingchemical reactions to crosslink polyolefin chains. Thus, in the presentmethods, the mixing of the peroxide is carried out in the solid state(not the melt state) and retains the peroxide in itsunreacted/undissociated form. This is different from the SSSP describedin U.S. Pat. No. 9,388,256 in which benzoyl peroxide is intentionallydissociated into free radical-containing fragments during SSSP. Theexistence of unreacted peroxide (and the resulting lack of polyolefincrosslinking) may be confirmed using differential scanning calorimetry(DSC) as described in the Example, below. SSSP conditions to achievedispersed and unreacted peroxide are described in the Example below. Inaddition, the peroxide may be added at a later stage to the solid-stateshear pulverizer, i.e., after an initial mixing of the polyolefin andthe unmodified graphite, to prevent its dissociation.

The result of mixing the polyolefin, the unmodified graphite, and theperoxide via SSSP as described above is a polymer nanocompositeprecursor composition comprising the polyolefin; exfoliated, unmodifiedgraphite dispersed throughout the polyolefin; and unreacted peroxidedispersed throughout the polyolefin. As further described below, thisprecursor composition has versatile melt processability and may be usedto provide crosslinked polyolefin nanocomposites having surprisinglyhigh wear resistance.

Although other polymers may be used, generally, the polymer of thepolymer nanocomposite precursor composition is a polyolefin. Thepolyolefin may be a homopolymer or a copolymer. Polyethylene andpolyethylene copolymers may be used. Combinations of different types ofpolyolefins may be used. In embodiments, the polyolefin is low-densitypolyethylene (LDPE) having a density in a range of rom 0.910 g/cm³ to0.940 g/cm³. The polyolefin generally comprises the bulk of theprecursor composition, with the amounts of unmodified graphite andunreacted crosslinker as described below.

Although other nanoscale fillers may be used, generally, the nanoscalefiller of the polymer nanocomposite precursor composition is unmodifiedgraphite. The term “unmodified” means that the graphite that is used inthe present methods is as received, without any pretreatment, asdescribed in the Example below. The unmodified graphite is exfoliatedand dispersed as described above. The amount of the unmodified graphitein the precursor composition may be selected to achieve a desiredproperty, e.g., maximum wear resistance. Illustrative amounts includefrom 0.1 weight % to 10 weight %, from 1 weight % to 7 weight %, andfrom 2 weight % to 5 weight %. (The term “weight %” refers to the weightof the unmodified graphite relative to the total weight of the precursorcomposition.) In embodiments, no other filler is used in the precursorcomposition other than the unmodified graphite.

Although other crosslinkers may be used (provided the crosslinker is onecapable of inducing chemical reactions to crosslink polymer chains), inembodiments, the crosslinker of the polymer nanocomposite precursorcomposition is a peroxide. As noted above, the peroxide is in itsunreacted form. This means that all the peroxide in the precursorcomposition is in its unreacted form or that the amount of dissociatedperoxide/free radical-containing fragments is too small to have amaterial effect. Similarly, all the polyolefin in the precursorcomposition is in its uncrosslinked form (i.e., all or an amount ofcrosslinked polyolefin too small to have a material effect). Peroxideshaving high dissociation temperatures (temperature at which the peroxidedissociates into free radical-containing fragments) may be used.Illustrative such peroxides include dicumyl peroxide; cumenehydroperoxide; t-butyl peroxide;2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane;2,5-Bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne; andBis[1-(tert-butylperoxy)-1-methylethyl]benzene. The amount of theperoxide in the precursor composition may be selected to achieve adesired property, e.g., maximum wear resistance. Illustrative amountsinclude from 0.1 weight % to 10 weight %, from 1 weight % to 7 weight %,and from 2 weight % to 5 weight %. (The term “weight %” refers to theweight of the peroxide relative to the total weight of the precursorcomposition.)

Although additives may be included in the polymer nanocompositeprecursor composition, in embodiments, the precursor compositionconsists of the polyolefin, the unmodified graphite, the unreactedperoxide, and optionally, one or more of a dye, a preservative, and anantioxidant.

Crosslinked polymer (e.g., polyolefin) nanocomposites may be formed bysubjecting any of the disclosed polymer nanocomposite precursorcompositions to a melt processing technique under conditions to inducechemical reactions to crosslink polyolefin chains (i.e., to inducecuring). A variety of existing melt processing techniques may be used,including compression molding, rotational molding, melt extrusion,injection molding, and powder coating. The conditions may be selected toachieve a desired degree (e.g., maximum) of crosslinking. For example,DSC may be used to confirm that the conditions fully convert theprecursor composition to its crosslinked/cured state as described in theExample below. (See FIG. 7 ; 100% conversion is full conversion and isindicative of maximum crosslinking.)

The crosslinked polymer nanocomposites may be characterized by a varietyof properties including non-isothermal crystallization onset temperature(T_(c, onset)), percent crystallinity, Young's modulus, ultimatestrength, elongation at break, and wear resistance as described in theExample below. However, as noted above, embodiments of the crosslinkedpolymer nanocomposites are characterized by surprisingly high wearresistance. Wear resistance may be quantified by a reduction in wearvolume as compared to that of a comparative material. By “comparativematerial” it is meant one formed using the same melt processingtechnique and conditions as the crosslinked polymer nanocomposite andfrom the same precursor composition except free of the nanoscale fillerand free of the crosslinker. For example, a crosslinked LDPEnanocomposite comprising crosslinked LDPE and exfoliated, unmodifiedgraphite dispersed therein may be compared against neat LDPE which hasbeen subjected to the same melt processing technique/conditions used toform the crosslinked LDPE nanocomposite. The tribology tests describedin the Example below may be used to determine reduction in wear volume.In embodiments, the crosslinked polymer nanocomposite is characterizedby a reduction in wear volume of at least 80%, at least 85%, or at least90% as compared to a comparative material. (See FIGS. 4B and 5 .)

Example Introduction

In this Example, the synergistic effects of crosslinking and fillerloading on wear resistance enhancement in crosslinked LDPE/graphitenanocomposites was demonstrated. Solid-state shear pulverization (SSSP)was used to prepare crosslinked nanocomposite powder precursorcontaining as-received graphite and undissociated dicumyl peroxide(DCP). SSSP is a tunably mild, continuous and scalable technique thatprocesses material in the solid state. The pulverizer is a modifiedtwin-screw extruder equipped with a cooling system to maintain thebarrel temperature below the melting temperature of semi-crystallinepolymer. During processing, the pulverizer provides effective sizereduction of filler aggregates and polymer powders, along with intimatemixing between components. The LDPE/graphite nanocomposite powderprecursor is further consolidated and crosslinked by melt processing.The precursor powder containing well-dispersed, unreacted DCP hasversatile melt processability with techniques ranging from compressionmolding and rotational molding to melt extrusion and injection molding.In stark contrast, mixing organic peroxide in the molten state viaconventional melt processes, e.g., melt extrusion or batch melt mixing,limits the further melt processability. This is believed to be due tocrosslinks developed during the mixing step.

Graphite was selected as the nanofiller. Some studies have used graphiteor graphene as filler to improve wear performance of nanocomposites, butpre-modification has been used. For example, synthetic steps have beenused to chemically treat the graphite in order to introduce functionalgroups, e.g., alkyl, onto the surface thereof. In contrast, SSSP is usedto significantly exfoliate as-received (unmodified) graphite intonanoplatelets containing several to ˜30 graphene layers and effectivelydisperse the nanoplatelets into the polymer matrix. The crosslinkingdegree is tuned by the amount of DCP incorporated during SSSP prior tomelt processing. This Example reports the wear rate of crosslinkedLDPE/graphite nanocomposites made by SSSP as a function of fillerloading and crosslinking degree. The uniaxial tensile properties andnon-isothermal crystallization behavior of these nanocomposites is alsoreported.

Experimental Materials

Low density polyethylene was provided by Exxon-Mobil (density=0.919g/cm³, MFI=1.1 g/10 min at 190° C.) and used as received. Dicumylperoxide and ultra-high molecular weight polyethylene (M_(w)=3,000-6,000kg/mol as reported by the supplier) was purchased from Sigma-Aldrich andused as received. As-received graphite was provided by Asbury Carbon(average flake diameter=2 μm, surface area=113 m²/g, as reported by thesupplier) and used without pretreatment.

Preparation of LDPE/Graphite Nanocomposites Powder Precursor

Solid-state shear pulverization was employed to prepare well-dispersedand well-exfoliated LDPE/graphite nanocomposite precursor powder, asshown in FIG. 1 . The pulverizer is a modified twin-screw extruder(Berstorff ZE-25) cooled by recirculating an ethylene glycol/water mixat 7° C. (Budzar Industries WC-3 chiller). Pellets of LDPE werecontinuously fed into the pulverizer via a pellet feeder (K-tron S-60)at 100 g/h. As-received graphite was added via a powder feeder(Brabender Technology Inc. DDSR12-1 volumetric feeder) at a feeding ratedepending the desired filler level (1-7 wt %) in the finalnanocomposites. The LDPE pellets and graphite were processed at 200 rpmscrew speed using a harsh screw design to achieve high levels of fillerdispersion. (See P. J. Brunner, et al., Polym. Eng. Sci. 2012; 52: 1555for detailed screw design and energy input information.) The powdermixture output was further processed for another four times via SSSP toachieve extensive graphite exfoliation and dispersion. The DCP was addedonly in the final pass, which suppresses/prevents any mechanochemicaldecomposition of the DCP resulting from shear forces. 1 or 3 wt % of DCPrelative to LDPE in the nanocomposite was added in order to maintain thesame crosslinking density regardless of graphite content.

Preparation of Linear or Crosslinked LDPE/Graphite Nanocomposites ViaCompression Molding

LDPE/graphite nanocomposites powder was consolidated into sheets bycompression molding. Powder precursor was pressed into a disc using apress. The disc was further sandwiched between Kapton films andcompression molded into a sheet (˜1 mm thick) using a PHI (mold 0230C-X1) press at 160° C. for 30 min with a 10-ton ram force followed byimmediately cooling in a cold press at 10° C. under 10-ton ram force.The crosslinking reaction induced by DCP is completed at 160° C. for 30min as monitored by differential scanning calorimetry (DSC); see FIG. 7. To measure conversion, the isothermal curing experiments wereconducted by holding the sample at 140° C. or 160° C. The heat flow wasrecorded as a function of time. The conversion of reaction wascalculated by integration of reaction heat at a given time divided bythe total heat during reaction. The heat of reaction during curing was14.5 mJ/mg in the 3DCP-LDPE sample.

Characterization of LDPE/Graphite Nanocomposites

The crystallization behavior of LDPE/graphite nanocomposites wascharacterized using a Mettler-Toledo differential scanning calorimeter(DSC 822e). The non-isothermal crystallization onset temperature(T_(c,onset)) was obtained using a 10° C./min cooling ramp from abovethe melt temperature. The percentage crystallinity was determined bydividing the sample enthalpy of fusion by the enthalpy of fusion for100% crystalline PE (285.9.1 J/g).

Uniaxial tensile testing samples were cut from sample sheets using aDewes-Gumbs die. Samples were equilibrated at room temperature for 48 hand then tested (ASTM D1708) using an MTS Sintech 2S tensile testerequipped with 5 kN load cell at a cross head speed of 50 mm/min. Atleast five specimens were tested for each sample.

X-ray diffraction was performed on a STOE-STADI-P diffractometer,operating with a Cu Kα radiation at 40 kV and 40 mA on ˜1 mm thickcompression molded films or graphite powder.

A reciprocal ball-on-disk tribometer (Rtec MFT-5000) was used toinvestigate the wear resistance of LDPE/graphite nanocomposites.Nanocomposite sheets were taped onto glass substrates. Tribologicaltests were carried out at a load of 5 N and a frequency of 3 Hz. Thesliding distance was 7.2 mm per cycle. The ball is AISI 316 stainlesssteel with a diameter of 0.25 inch. A virgin stainless steel ball wasmounted before each test. Each test was conducted for 1 hr at roomtemperature and humidity without any lubrication. Samples were rinsedwith ethanol to remove the wear debris after tribological tests. Thedepth profile of the wear track was obtained via a 3D laser confocaloptical microscope (Olympus OLS5000), thus allowing determination of thewear volume. The depth profile of the wear track cross section wasaveraged over 300 μm in length along the wear direction.

Results and Discussion

Filler Dispersion in LDPE/Graphite Hybrids and Nanocomposites (withoutCrosslinking) Determined by X-Ray Diffraction (XRD)

XRD was used to reveal the macroscopic extent of as-received graphiteexfoliation in the LDPE/graphite nanocomposites. FIG. 2 shows the XRDdata for as-received graphite, neat LDPE (top), LDPE/3G (middle, 3Grefers to the 3 wt % of graphite relative to LDPE) hybrid prepared bymelt mixing in a cup-and-rotor mixer, and LDPE/3G nanocomposite preparedby SSSP (bottom). The data for neat LDPE and hybrids were normalized bymaking the areas associated with PE crystal peaks equal. (Neat LDPE andthe LDPE hybrids and nanocomposites have the same crystallinity withinexperimental error as determined by DSC.) As-received graphite exhibitsa diffraction peak at 2θ=26.5°, corresponding to an average interlayerspacing (d-spacing) of 0.336 nm according to Bragg's law. This value isin excellent accord with the d-spacing of natural graphite (0.335 nm, B.T. Kelly, Physics of Graphite, Applied Science: London, 1981.). NeatLDPE and the hybrid/nanocomposites exhibit peaks at 2θ=21.3° and2θ=23.7°, corresponding to the diffracted X-ray intensity from (110) and(200) planes, respectively (S. Aggarwal, et al., J. Polym. Sci. 1955;18: 17.). The inset of FIG. 2 shows the XRD data for neat LDPE and LDPEhybrid/nanocomposite in the 2θ range of 25° to 28°. Neat LDPE exhibitsessentially no diffraction peak in this region. The LDPE/3G hybridprepared by melt mixing exhibits a sharp peak associated with graphite.Thus, the relatively weak shear forces provided by conventional meltmixing are insufficient to substantially exfoliate as-received graphitein the LDPE matrix. In contrast, the LDPE/3G nanocomposite prepared viaSSSP exhibits a very strongly suppressed X-ray peak associated withunexfoliated graphite. These results indicate that very substantiallevels of graphite exfoliation are achieved during SSSP. As thisgraphite exfoliation achieved by SSSP occurs prior to any crosslinking,the highly exfoliated state of the nanocomposites should be retainedafter compression molding the nanocomposites containing DCP, leading tocrosslinked LDPE/graphite nanocomposites.

Crystallization Behavior of LDPE/Graphite Nanocomposites Prepared bySSSP (with and without Crosslinking)

FIG. 3 and Table 1 show the non-isothermal crystallization behaviors ofneat LDPE, 1DCP-LDPE, 3DCP-LDPE (1DCP and 3DCP refers to 1 wt % and 3 wt% of DCP relative to LDPE, respectively) and correspondingnanocomposites containing 1 to 7 wt % graphite. Crystallization onsettemperature (T_(c,onset)) and crystallinity were obtained from DSCmeasurements taken upon cooling from the melt at a 10° C./min coolingrate. 1 wt % and 3 wt % DCP relative to LDPE is incorporated during SSSPto yield lightly crosslinked 1DCP-LDPE and highly crosslinked 3DCP-LDPE,respectively. 1DCP-LDPE and 3DCP-LDPE exhibit T_(c,onset) values thatare 3° C. and 10° C. lower than that of neat LDPE, as shown in FIG. 3 .The mobility of crosslinked LDPE in the molten state is restricted bythe permanent crosslinks. As a result, those restricted polymer chainslead to lower T_(c,onset) values. The crystallinities of 1DCP-LDPE and3DCP-LDPE are 3% and 6% lower than that of neat LDPE, respectively.These reductions are consistent with the notion that that the ethylenerepeat units closest to the crosslinks are not able to fold intolamella. Thus, higher crosslinking density leads to lower crystallinityin these crosslinked LDPE materials.

TABLE 1 Crystallization behavior of neat LDPE and LDPE/graphitenanocomposites prepared by SSSP. LDPE 1DCP-LDPE^(a) 3DCP-LDPE^(a) FillerCrystal- Crystal- Crystal- content T_(c. onset) linity ^(b) T_(c. onset)linity ^(b) T_(c. onset) linity ^(b) (wt %) (° C.) (%) (° C.) (%) (° C.)(%) 0 97 44 94 41 87 38 1 101 43 96 40 89 37 3 101 44 98 40 90 37 5 10142 99 40 92 37 7 101 42 99 40 92 37 ^(a)1DCP and 3DCP refers to 1 wt %and 3 wt % of DCP relative to LDPE, respectively. ^(b) Crystallinityrefers to the PE percentage crystallinity determined upon cooling fromthe melt at a 10° C./min cooling rate.

As shown in Table 1, all LDPE/graphite nanocomposites exhibitessentially the same crystallinity as the neat LDPE matrix or DCP-LDPEmaterial within a small error. The addition of 1 wt % well exfoliatedgraphite in LDPE leads to a 4° C. increase in T_(c,onset) values. Thisis because the well-exfoliated nanofiller with enormoussurface-to-volume ratio can serve as effective heterogeneous nucleationagents for LDPE crystallization. With increasing filler content, theT_(c,onset) values of LDPE/graphite nanocomposites remain invariant.This is due to the fact that LDPE is a highly crystallizablesemi-crystalline polymer, and a certain degree of supercooling isrequired for the formation and growth of crystallites. In contrast,T_(c,onset) values of 1DCP-LDPE/graphite nanocomposites increasedgradually from 96° C. to 99° C. with increasing filler loading. Similarbehavior is observed in 3DCP-LDPE/graphite nanocomposites. Thecrosslinkers restrict the chain mobility and thus 1DCP-LDPE and3DCP-LLDPE are less crystallizable than neat LDPE. The presence ofheterogeneous nucleation sites facilitates the formation and growth ofcrystallites. As a result, the T_(c,onset) values of crosslinked LDPEnanocomposites increase with increasing amounts of heterogeneousnucleation sites.

Uniaxial Tensile Behavior of LDPE/Graphite Nanocomposites

Table 2 shows the uniaxial tensile properties of LDPE/graphitenanocomposites as a function of filler loading and crosslinking density.The Young's modulus of neat LDPE and 1DCP-LDPE are the same withinexperimental error. In semi-crystalline PE, the Young's modulus ofcrystalline phase is on the order of ˜200 GP, and the apparent Young'smodulus of LDPE decrease with decreasing crystallinity. (See I.Sakurada, et al., J. Polym. Sci. 1962; 57: 651. J. Clements, et al.,Polymer 1978; 19: 639, and J. Halpin, et al., J. Appl. Phys. 1972; 43:2235.) On the other hand, according to ideal rubber elasticity theory,the Young's modulus of crosslinked polymer scales linearly withcrosslinking density (P. J. Flory, Principles of Polymer Chemistry,Cornell University Press: Ithaca, N.Y., 1953). In lightly crosslinked1DCP-LDPE, the introduction of crosslinks compensates the slightlylowered crystallinity relative to neat LDPE, leading to invariantYoung's modulus. Although 3DCP-LDPE has higher crosslinking density, theYoung's modulus is reduced by 48% relative to the neat LDPE due to lowercrystallinity and thinner lamella structure. Relative to neat LDPE,1DCP-LDPE and 3DCP-LDPE, at 7 wt % graphite content, the Young's moduliof the nanocomposites was enhanced by 62%, 82% and 82%, respectively.

TABLE 2 Uniaxial tensile properties of PE/graphite nanocomposites.Young's modulus Ultimate strength Elongation at Sample name ^(a) (MPa)(MPa) break (%) LDPE 210 ± 20  8.8 ± 0.3 660 ± 40 LDPE/1G 270 ± 10  9.3± 0.5 530 ± 80 LDPE/3G 290 ± 20  9.9 ± 0.2 550 ± 70 LDPE/5G 310 ± 20 9.8 ± 0.4 460 ± 30 LDPE/7G 340 ± 30 10.3 ± 0.2 410 ± 30 1DCP-LDPE 220 ±10 18.2 ± 1.6 650 ± 40 1DCP-LDPE/1G 240 ± 10 18.9 ± 0.5 610 ± 201DCP-LDPE/3G 270 ± 20 18.7 ± 1.6 630 ± 40 1DCP-LDPE/5G 280 ± 10 16.6 ±0.6 560 ± 30 1DCP-LDPE/7G 300 ± 20 15.1 ± 1.1 470 ± 70 3DCP-LDPE 110 ±20 15.1 ± 1.3 420 ± 30 3DCP-LDPE/1G 150 ± 20 15.7 ± 1.5 380 ± 303DCP-LDPE/3G 150 ± 10 17.2 ± 0.7 400 ± 20 3DCP-LDPE/5G 160 ± 20 18.7 ±1.9 450 ± 50 3DCP-LDPE/7G 200 ± 30 18.3 ± 2.6 470 ± 60 ^(a) LDPE/1Grefers to 1 wt % of graphite relative to LDPE. 1DCP and 3DCP refers to 1wt % and 3 wt % of DCP relative to LDPE, respectively.

The elongation at break is reduced from 660% in neat LDPE to 420% in3DCP-LDPE (without any filler). The incorporation of chemical crosslinksimposes the restriction of chain mobility during deformation, reducingthe probability of chain slipping. As a result, the elongation at breakin highly crosslinked 3DCP-LDPE is mainly determined by the averagestretchability of chains between the chemical crosslinks. The ductilityof lightly crosslinked 1DCP-LDPE is the same as neat LDPE within errordue to the low degree of crosslinking density. The incorporation ofrigid nanofiller in neat LDPE is expected to reduce the ductility due tothe defects caused by filler agglomerates. For example, the elongationat break is reduced from 660% in neat LDPE to 460% in LDPE/5Gnanocomposites. Even at 7 wt % graphite loading with 38% reducedelongation at break relative to neat LDPE, the nanocomposites stillremain ductile. In contrast, the elongation at break in3DCP-LDPE/graphite nanocomposites remain invariant with increasingfiller loading within error. Due to the restricted mobility imposed bychemical crosslinks, the elongation at break of 3DCP-LDPE/graphitenanocomposites is mainly determined by the ductility of matrix polymer.Thus, the defects caused by nanofiller aggregates will not significantlyaffect the elongation at break, even at 7 wt % filler loading.

Wear Resistance of LDPE/Graphite Nanocomposites

Wear track profiles were obtained using a 3D laser confocal opticalmicroscope, with representative results shown in FIGS. 4A-4C. FIG. 4Apresents the top view image of a wear track on neat LDPE after thetribology test. The round edges at the end of wear track were excludedfor wear volume calculation; only the middle section (4 mm in lengthalong the wear direction) was used. Scanning the wear track over a 4138μm×1226 μm area in the middle results in the 3D depth profile shown inFIG. 4B. FIG. 4C shows the wear track depth profile averaged over 300 μmin length along the wear direction in neat LDPE, 1DCP-LDPE and 3DCP-LDPEsamples. The hatched region in the profile is taken as the average crosssection area for wear volume calculation.

FIG. 5 shows the wear volume of samples as a function of graphiteloading and crosslinking degree calculated using the cross-sectionprofiles determined above. Relative to neat LDPE, lightly crosslinked1DCP-LDPE and highly crosslinked 3DCP-LDPE (without filler) exhibit 20%and 83% reductions in wear volume, respectively. The enhancement of wearperformance can be attributed to additional chemical crosslinks relativeto neat LDPE. The low degree of physical crosslinks in LDPE leads toaccelerated abrasive wear involving removal of submicron particles fromthe contacting surfaces under wear conditions. Highly crosslinked LDPEis analogous to PE with infinitely high molecular weight. These chemicalcrosslinks can effectively compensate for the insufficient physicalcrosslinks and significantly enhance the wear resistance. Theincorporation of well-dispersed graphite significantly reduces the wearvolume in nanocomposites up to 3 wt % loading. For instance, LDPE/3G,1DCP-LDPE/3G and 3DCP-LDPE/3G exhibits 54%, 47% and 28% reduced wearvolume relative to corresponding neat matrices, respectively. Thisbehavior can be rationalized by the fact that graphite is aself-lubricating material, the inter-layer sliding motion of graphitenanoplates containing several to ˜30 graphene layers during thetribology test could significantly enhance the wear performance ofnanocomposite materials. The large interfacial area between the PEmatrix and the well-dispersed graphene nanoplatelets provides strongpolymer-filler interactions, which facilitate the stress transfer to thefiller network. Better stress transfer to filler during the slidingmotion could effectively improve the wear resistance of graphenenanocomposites relative to the neat polymer matrices. The reduced wearvolume can also be attributed to the enhanced mechanical properties ofnanocomposites relative to neat PE matrices. As shown in Table 2,LDPE/3G, 1DCP-LDPE/3G and 3DCP-LDPE/3G exhibits Young's modulus valuesthat are 38%, 23% and 36% higher than corresponding neat matrices,respectively. Although not directly measured, the shear modulus is alsoenhanced as observed by the enhanced Young's modulus. The nanocompositesexhibit a reduction of shear deformation relative to neat PE matrices,resulting in fewer PE chains abraded from the contacting area. A furtherincrease of graphite from 3 wt % to 7 wt % does not reduce wear volumewithin error. This behavior may be explained by that fact that fillerdispersion quality may not be maintained at a constant level at a higherfiller loading level.

The incorporation of the combination of chemical crosslinks andwell-dispersed graphite substantially enhances the wear resistance ofthe crosslinked LDPE nanocomposites relative to neat LDPE. The strengthsof these two factors were further compared. LDPE/3G (without crosslinks)exhibits a wear volume of 0.18 mm³, 54% less than that of neat LDPE,after the tribology test. In stark contrast, crosslinked 3DCP-LDPE(without the incorporation of any graphite) has a wear volume of only0.06 mm³, 83% less than that of neat LDPE. These results indicate thatchemical crosslinks play a stronger role than nanofiller in reducing thewear volume. This may be rationalized from the notion that changing thePE architecture from poorly entangled neat LDPE to covalentlycrosslinked LDPE, the wear resistance will be strongly enhanced becausethe LDPE makes up the vast majority of the nanocomposite material. Incontrast, graphite filler makes up only a few wt % of the nanocompositesand can only lubricate the counterface and alleviate the wear damage. Inthe optimum circumstance encountered here, 3DCP-LDPE with 3 wt % orhigher graphite filler loading exhibits a wear volume reduction of ˜88%relative to neat LDPE. Notably, a commercial UHMWPE sample (3,000-6,000kg/mol reported by supplier), which is expected to have a vastlyimproved wear resistance relative to neat LPDE, exhibits a wear volumethat is a factor of 3.5 smaller than that of 3DCP-LDPE/3G crosslinkednanocomposite under the same experimental conditions. (See FIG. 8 fordepth profile of the wear track cross section.) Moreover, in comparisonwith UHMWPE, the nanocomposite powder precursors provide much moreversatile melt processability into final products, including extrusion,injection molding, powder coating and rotational molding.

FIG. 6A shows the variation of coefficient of friction (COF) during weartests in neat LDPE, 1DCP-LDPE, 3DCP-LDPE and LDPE/3G. The COF wasaveraged over the data range after 10 min to avoid the large variationin the initial “break-in” stage. It is noted that there is a minorincrease of COF in neat LDPE and 1DCP-LDPE as a function time. Thisbehavior could be due to the fact that the contacting area between thestainless-steel ball and sample surface increases with increasing weartrack depth, leading to larger frictional forces along the slidingdirection and a higher COF. Such an increase is negligible in 3DCP-LDPEbecause the wear track is much shallower as compared to neat LDPE and1DCP-LDPE.

As shown in FIG. 6A, the COF increases with increasing crosslinkingdegree. For instance, the average COF increased from 0.23 in neat LDPEto 0.28 in 3DCP-LDPE. This phenomenon has been previously observed incrosslinked HDPE with the COF increasing with increasing crosslinkdensity. Under the constant sliding motion of metal balls against thepolymer surface, a transfer film of polymer powder would form to shieldthe soft polymer from the counterface. As most polymers areself-lubricating materials, the transfer layer serves as a lubricant toreduce the COF. Such a transfer layer was observed in neat linear HDPEsupported by a layer of HDPE deposited on the metal surface. However, notransfer film was observed in crosslinked HDPE. As a result, the metalball directly rubs against the crosslinked HDPE, leading to a higher COFrelative to linear HDPE. These results are sufficient to explain thelarger COF in LDPE with higher crosslinking degree in this currentstudy. The incorporation of 3 wt % well-dispersed graphite in neat LDPEled to COF reduction from ˜0.23 to ˜0.18. The existence ofself-lubricating graphite in the transfer layer could effectively reducethe COF in LDPE/graphite nanocomposites.

FIG. 6B shows the average COF of LDPE/graphite nanocomposites as afunction of crosslinking density and graphite loading. In LDPE/graphitenanocomposite, the COF decreases with increasing filler loading. Instark contrast, the addition of graphite does not impact the COF in1DCP-LDPE/graphite and 3DCP-LDPE/graphite nanocomposites, within error.As described earlier, the transfer layer in neat LDPE serves as alubricant to reduce COF during tribology tests. The lower COF ofLDPE/graphite nanocomposites indicates a reduced abrasive wear relativeto neat LDPE, in accordance with the enhanced wear performance. However,in 1DCP-LDPE/graphite and 3DCP-LDPE/graphite nanocomposites, thestainless ball directly rubs against the sample in the absence of atransfer layer. The existence of graphite does not effectively lubricatethe wear track, leading to a COF that is invariant in crosslinkednanocomposites relative to corresponding neat matrices.

CONCLUSIONS

Crosslinked LDPE nanocomposite precursors containing as-receivedgraphite were prepared via SSSP with up to 7 wt % filler and up to 3 wt% crosslinking agent, dicumyl peroxide. The powder precursors werefurther consolidated and cured in the molten state by compressionmolding. X-ray diffraction results revealed high levels of graphiteexfoliation achieved by SSSP without the need for fillerpre-modification. Relative to neat LDPE, the non-isothermalcrystallization onset temperature, percentage crystallinity and Young'smodulus of crosslinked LDPE decreases with increasing crosslinkingdensity. The incorporation of well-dispersed and well-exfoliatedgraphite enhanced the non-isothermal crystallization onset temperature,Young's modulus and ultimate strength of nanocomposites relative tocorresponding neat PE matrices; in contrast, the polymer percentagecrystallinity was unaffected within experimental error. Theincorporation of chemical crosslinks and well-dispersed graphitesynergistically enhance the wear resistance of crosslinkednanocomposites relative to neat LDPE, with chemical crosslinks playingthe stronger role in reducing the wear rate. In the optimum circumstanceencountered here, 3DCP-LDPE/3G exhibited a wear volume that is 88% lessthan that of neat LDPE. Crosslinked LDPE/graphite nanocomposites providea simple method to improve the poor wear performance of neat LDPE andthus providing low-cost materials for high-end uses where wearresistance is important, e.g., coatings. Moreover, relative to UHMWPEwhich also provides high wear resistance, the nanocomposite precursorsprepared in our study exhibit much more versatile melt processability,including that involved in applications such as powder coating androtational molding.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more.”

If not already included, all numeric values of parameters in the presentdisclosure are proceeded by the term “about” which means approximately.This encompasses those variations inherent to the measurement of therelevant parameter as understood by those of ordinary skill in the art.This also encompasses the exact value of the disclosed numeric value andvalues that round to the disclosed numeric value.

The foregoing description of illustrative embodiments of the disclosurehas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the disclosure to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of thedisclosure. The embodiments were chosen and described in order toexplain the principles of the disclosure and as practical applicationsof the disclosure to enable one skilled in the art to utilize thedisclosure in various embodiments and with various modifications assuited to the particular use contemplated. It is intended that the scopeof the disclosure be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A polyolefin nanocomposite precursor compositioncomprising low-density polyethylene; exfoliated, unmodified graphitedispersed throughout the low-density polyethylene; and unreactedperoxide crosslinker dispersed throughout the low-density polyethylene.2. The polyolefin nanocomposite precursor composition of claim 1, havinga diffraction peak at 26.5° having a normalized intensity that is within5% of that of the low-density polyethylene without the exfoliated,unmodified graphite and without the unreacted peroxide crosslinker asmeasured by X-ray diffraction.
 3. The polyolefin nanocomposite precursorcomposition of claim 1, wherein the low-density polyethylene has adensity in a range of from 0.910 g/cm³ to 0.940 g/cm³.
 4. The polyolefinnanocomposite precursor composition of claim 1, wherein the unreactedperoxide crosslinker is dicumyl peroxide; cumene hydroperoxide; t-butylperoxide; 2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane;2,5-Bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne;Bis[1-(tert-butylperoxy)-1-methylethyl]benzene; or a combinationthereof.
 5. The polyolefin nanocomposite precursor composition of claim1, wherein the unreacted peroxide crosslinker is dicumyl peroxide. 6.The polyolefin nanocomposite precursor composition of claim 1, whereinthe exfoliated, unmodified graphite is present at an amount in a rangeof from 1 weight % to 7 weight % and the unreacted peroxide crosslinkeris present at an amount in a range of from 1 weight % to 7 weight %. 7.The polyolefin nanocomposite precursor composition of claim 1, whereinthe exfoliated, unmodified graphite is present at an amount in a rangeof from 2 weight % to 5 weight % and the unreacted peroxide crosslinkeris present at an amount in a range of from 2 weight % to 5 weight %. 8.The polyolefin nanocomposite precursor composition of claim 1,consisting of the low-density polyethylene, the exfoliated, unmodifiedgraphite, the unreacted peroxide crosslinker, and optionally, one ormore of a dye, a preservative, and an antioxidant.
 9. The polyolefinnanocomposite precursor composition of claim 8, wherein the low-densitypolyethylene has a density in a range of from 0.910 g/cm³ to 0.940 g/cm³and wherein the unreacted peroxide crosslinker is dicumyl peroxide;cumene hydroperoxide; t-butyl peroxide;2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane;2,5-Bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne;Bis[1-(tert-butylperoxy)-1-methylethyl]benzene; or a combinationthereof.
 10. The polyolefin nanocomposite precursor composition of claim9, wherein the exfoliated, unmodified graphite is present at an amountin a range of from 1 weight % to 7 weight % and the unreacted peroxidecrosslinker is present at an amount in a range of from 1 weight % to 7weight %.
 11. A crosslinked polyolefin nanocomposite comprisingcrosslinked low-density polyethylene and exfoliated, unmodified graphitedispersed throughout the crosslinked low-density polyethylene.
 12. Thecrosslinked polyolefin nanocomposite of claim 11, wherein theexfoliated, unmodified graphite is present at an amount in a range offrom 1 weight % to 7 weight %.
 13. The crosslinked polyolefinnanocomposite of claim 11, wherein the exfoliated, unmodified graphiteis present at an amount in a range of from 2 weight % to 5 weight %. 14.The crosslinked polyolefin nanocomposite of claim 11, exhibiting areduction in wear volume of at least 80% as compared to neat low-densitypolyethylene.
 15. The crosslinked polyolefin nanocomposite of claim 11,consisting of the low-density polyethylene, the exfoliated, unmodifiedgraphite, and optionally, one or more of a dye, a preservative, and anantioxidant.
 16. The crosslinked polyolefin nanocomposite of claim 15,wherein the exfoliated, unmodified graphite is present at an amount in arange of from 1 weight % to 7 weight %.
 17. The crosslinked polyolefinnanocomposite of claim 15, wherein the exfoliated, unmodified graphiteis present at an amount in a range of from 2 weight % to 5 weight %. 18.The crosslinked polyolefin nanocomposite of claim 15, exhibiting areduction in wear volume of at least 80% as compared to neat low-densitypolyethylene.
 19. A method of forming a crosslinked polyolefinnanocomposite, the method comprising subjecting a polyolefinnanocomposite precursor composition comprising a polyolefin; exfoliated,unmodified graphite dispersed throughout the polyolefin; and unreactedperoxide crosslinker dispersed throughout the polyolefin, wherein thepolyolefin is polyethylene, a copolymer of polyethylene, or combinationsthereof, to a melt processing technique under conditions to inducechemical reactions to crosslink chains of the polyolefin, therebyforming a crosslinked polyolefin nanocomposite.
 20. The method of claim19, wherein the polyolefin is low-density polyethylene.